1. Technical Field
The present invention relates to a transmission, particularly to a transmission capable of switching a transmission gear swiftly in response to operation of disengaging a clutch.
2. Background Art
There is known a transmission for engaging a shift fork to a cam groove formed at a surrounding of a shift drum and switching mesh of gear by moving the shift fork by pivoting the shift drum. In moving the shift fork, it is necessary to cut a power transmission path between a power source and a transmission of a vehicle and a clutch mechanism is adopted therefor. In recent years, there is known an electric transmission for cutting a power transmission mechanism by a clutch mechanism and driving a shift drum by a single motor (for example, refer to Japanese Unexamined Patent Publication No. H11-82710).
The electric transmission is provided with a change spindle for driving an arm (master arm) for intermittently pivoting the shift drum. The change spindle is also coupled with a clutch lever for disengaging and engaging a clutch. In operating thereof, when the change spindle is rotated by a predetermined angle, the clutch is disengaged via the clutch arm and when the change spindle is rotated further from the position, the shift drum is rotated, the shift fork is displaced and mesh of gear is shifted.
Conventionally, the shift drum is not rotated until the clutch is disengaged after the change spindle is rotated by the predetermined angle. Therefore, a time period from start of shift operation until the gear is shifted actually is prolonged. Further, a dowel is formed in a connecting member for connecting gears moved by the shift fork, slits with which the dowel engages are formed at the gears and a transmission gear ratio is determined by to which slits of gear the dowel of the connecting member is coupled.
The dowel and the slits are provided with a structure in which the dowel and the slit are smoothly coupled when the clutch is disengaged and rotation by inertia is continued. However, in the conventional apparatus for rotating the shift drum after the clutch is disengaged, a time period from when the clutch is disengaged until operation of coupling the dowel and the slit is reached, is long. Therefore, the inertia is small, the dowel and the slit are difficult to couple and the gear may not be shifted smoothly.
There is proposed a shift mechanism in which prior to shift operation the shift fork is urged in a direction of displacing the shift fork by a spring (refer to Japanese Unexamined Utility Model Publication No. S62-40349, Japanese Unexamined Patent Publication No. H5-203048. However, according to these shift mechanisms, the shift drum is not used but the shift fork is directly driven and therefore, a motor must be provided exclusively for driving the shift fork.
It is an object of the present invention to resolve the above-described problem and provide a transmission capable of swiftly shifting gear by moving a shift fork immediately when a clutch is disengaged.
In order to achieve the above-described object, the present invention is characterized firstly by a transmission for changing a ratio of a rotational number of a power source and a rotational number of an output shaft by shifting a gear train provided between the power source and the output shaft by moving a shift fork, the transmission comprising a shift drum for moving the shift fork, a clutch provided between the power source and the gear train, a shift spindle for urging the shift drum and the clutch, intermittent driving means provided between the shift spindle and the shift drum for starting to operate from a first operation angle of the shift spindle and pivoting the shift drum by a predetermined angle and urging means provided between the shift spindle and the shift drum for starting to urge the shift drum in a pivoting direction at a third operation angle smaller than a second operation angle of the shift spindle at which disengagement of the clutch is finished.
According to the first aspect, when the shift spindle is pivoted, the clutch is disengaged at the second operation angle. Further, prior to disengaging the clutch, by the urging means, preload starts applying to the shift drum from the third operation angle. Therefore, when the clutch is disengaged, the shift drum is immediately pivoted by the predetermined angle and the gear train is shifted.
Further, the present invention is secondly characterized in that urging force by the urging means is released at an operation angle larger than the first operation angle. According to the second aspect, after reaching the first operation angle, that is, after starting to pivot the shift drum by the intermittent driving means, the shift drum follows pivoting of the shift spindle.
Further, the present invention is thirdly characterized in further providing drum position detecting means for detecting a pivoting angle of the shift drum. According to the third aspect, even when the shift drum is rotated by the urging means before reaching the first operation angle, the position of the shift drum can be recognized and accordingly, various control based on the angle detection result can surely be carried out in correspondence with the position of the shift drum.
Further, the present invention is fourthly characterized in further providing a motor for driving the shift spindle. According to the fourth aspect, in the apparatus in which the clutch is disengaged and the gear is shifted by the motor, the clutch is switched surely in a short period of time.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.